1. Field of the Invention
The present invention relates to an optical apparatus such as a video camera or still camera having a forward/backward movable filter member in an optical path of an image taking optical system.
2. Related Background Art
When image taking an object with a still camera or video camera, if an aperture stop diameter becomes too small though the object is bright, deterioration of image quality due to diffraction and dust image taken due to increase of focal depth become issues. Therefore, to prevent an aperture stop diameter from becoming too small, there are some still cameras or video cameras in which an ND filter is directly set to an aperture stop vane of the aperture stop apparatus of an image taking lens, an aperture stop opening is formed by driving a plurality of aperture stop vanes, and the ND filter integrally set to the aperture stop vane is inserted into the aperture stop opening. Moreover, there are some cameras each of which is provided with an aperture stop apparatus in which an ND filter is forward/backward moved independently of aperture stop vanes of the aperture stop apparatus so that forward/backward movement of the ND filter can be automatically or manually changed.
Furthermore, there are some cameras in each of which the shape of an object can be seen even in nighttime by constituting an infrared cut filter so that it can be inserted or extracted and extracting the infrared cut filter from an optical path to take in a lot of infrared rays when the shape of the object cannot be easily seen in nighttime image taking case.
When inserting an optical member such as an ND filter into the optical path of an image taking optical system, a focus position is displaced depending on the difference of the refraction index of the optical member and the thickness of the member. Moreover, when removing an infrared cut filter normally used for image taking from an optical path, a focus position is displaced. Usually, focusing is instantaneously performed through autofocusing. However, when the brightness of an object is changed under manual focusing and the position of an ND filter in an optical path is changed, the ND filter is manually inserted or extracted, or an aperture stop opens by changing the present mode to the portrait mode, an unfocused image may be obtained because a filter insertion state is changed.
According to decrease in size of cameras and improve of an image quality in recent years, when CCDs are decreased in size and increased in the number of pixels, because of decreasing a pixel pitch, a focus displacement due to the thickness of an ND filter cannot be ignored though the influence of the thickness of the ND filter has not been great so far.
Japanese Patent Application Laid-Open No. 11-289487 discloses a configuration in which an ND filter is set separately from aperture stop vanes of an aperture stop apparatus and the ND filter is inserted or extracted into or from an optical path separately from driving of the aperture stop vanes. Moreover, the configuration is constituted so as to detect the insertion amount of the ND filter into an optical path, drive a focus lens by a correction amount corresponding to the insertion amount, and correct a focus displacement. Furthermore, Japanese Patent Application Laid-Open Nos. 11-72691 and 11-305107 disclose a configuration in which an optical filter is set separately from aperture stop vanes of an aperture stop apparatus to completely insert the optical filter into or completely remove the filter from an optical path separately from driving of the aperture stop vanes. In the disclosed configuration, zoom position detecting means is included to drive a focus lens depending on whether an ND filter or infrared cut filter is present in an optical path and a focus lens is corrected by a correction amount conforming to a zoom position.
In the case of Japanese Patent Application Laid-Open No. 11-289487, though a focus lens correction amount corresponding to an insertion amount of an ND filter is used, there is a problem that an accurate correction amount cannot be obtained by only the insertion amount of the ND filter because the rate of the area covered with the ND filter in the opening area of an optical path depends on an opening diameter of an aperture stop. Moreover, in the case of Japanese Patent Application Laid-Open Nos. 11-72691and 11-305107, there is a problem that an accurate correction amount cannot be obtained because a state is assumed in which an optical component such as an ND filter completely covers or completely removes from an optical path but image taking when the ND filter half covers the optical path is not considered.